Pipe couplings are commercially available from a variety of manufacturing sources and are used extensively for underground pipe installations as in the distribution of natural gas. It is known in such couplings to employ gripping members of sorts in order to prevent in-service uncoupling of the joint by longitudinal drawbar forces imposed on the coupled pipe section.
With the advent of plastic pipe and tubing and their approval by regulating governmental agencies for natural gas transmission, the previous problem of pipe pullout from a coupled joint has increased many times over to on the order of tenfold as compared to metal pipe. One factor most contributing to this problem is the much greater linear coefficient of thermal expansion for plastic pipe as compared to steel pipe. By way of example, Aldyl "A" polyethylene piping marketed by Dupont has a linear coefficient of thermal expansion stated by the manufacturer to be: 9.times.10.sup.-5 in/in. F.degree. as compared to 6.5.times.10.sup.-6 in/in. F.degree. for steel pipe. Notwithstanding, government agencies regulating installation of such systems require that each joint sustain the longitudinal pullout or thrust forces caused by contraction or expansion of the piping or by anticipated external or internal loading to within prescribed limits. (Title 49, Section 192, "Transportation of Natural and Other Gas by Pipeline-Minimum Safety Regulations"--Fed. Vol. 35, No. 161).
While the degree of pipe restraint or lockup imposed on the coupled joint is critical, it is at least equally critical that adequate gasket compression be achieved about the coupled pipe in the course of coupling installation to insure a seal against leakage of line content. A typical bolt type coupling construction utilized in the prior art for natural gas service comprises the combination of a gasket to effect sealing and a lock ring to restrain the pipe against pullout. Such couplings are operationally characterized by a grip imposed on the pipe by the lock ring that normally occurs before or substantially simultaneous with attainment of the required gasket pressure for sealing.
To the extent pipe grip by the lock ring occurs before gasket pressure is achieved, some slight displacement of the pipe ends may occur relative to each other in the course of completing the gasket seal. There are, however, certain installations where, for example, the pipe ends are butting or restrained such as at a final tie-in that preclude pipe displacement to enable completion of the gasket seal. In those situations, should the prior coupling construction effect lockup before the required gasket seal is obtained about the coupled pipe, the required gasket seal may not be obtained.